Pressure control system for



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Bar: 15' figz/file 4702x454 I Reissued Aug. 26, 1952 PRESSURE CONTROL SYSTEM FOR AIRCRAFT CABINS Bruce E. Del Mar, Los Angeles, Calif., assignor to Douglas Aircraft Company, Inc., Santa Monica.

Calif.

Original No. 2,549,673, dated April 17, 1951, Serial No. 669,366, May 13, 1946. Application for reissue February 23, 1952, Serial No. 273,105 v Matter enclosed in heavy brackets I: appears in the original patent but forms no part of this reissue specification; matter printed in italics indicates the additions made by reissue.

42 Claims.

This invention relates to the control of pressure in sealed cabins such as those of aircraft in order to isolate flight personnel from uncomfortable changes in pressure and low values of absolute pressure experienced at high altitudes.

It is well known that pressure changes encountered in flight may cause considerable discomfort, and that the low pressures encountered at higher altitudes'necessarily limit the flight altitudes of aircraft not provided with supercharged cabins. Considerable development has been undertaken in the past toward providing means to control the pressure in aircraft cabins along certain predetermined schedules.

It was soon found that it was not practical to maintain the absolute pressure in the cabin at values normally encountered at airport levels due to extreme penalties in weight and power. Furthermore, it was found that passengers could readily tolerate the lower absolute pressure corresponding to an intermediate altitude somewhat above airport levels. Initial efforts were directed toward simply maintaining cabin pressure constant at an intermediate value whenever flight above that prescribed level was attempted and in conjunction therewith. a schedule of maximum pressure difference between the cabin and the flight atmosphere was prescribed at a convenient value so that whenever flight was attempted above a predetermined level. the absolute pressure in the cabin thereafter would decrease and maintain the pressure difference between cabin interior and exterior constant at a safe limit differential pressure value.

Control of cabin pressure at the constant value of an intermediate altitude was soon found impractical since it is obvious that little had been achieved in isolating the occupants from the rather rapid changes in pressure during flight up to and down from the prescribed control level which is in a region where the air is most dense and where the pressure thereof changes most rapidly with changes in altitude. To achieve results in this direction the next step taken was to add to the aforementioned control, a time rate of cabin pressure change control whereby cabin pressure could be set to increase or decrease in increments corresponding to a chosen rate of altitude change until the intermediate pressure value was attained on ascent, or until cabin pressure was equalized with flight pressure during descent. This mode of control has proven entirely inadequate as the operator must continually "fly" the cabin pressures by setting its controls not only at critical times upon starting ascent and descent of the aircraft, but also to reset these controls under continual observation during ascents and descents in order to attempt to isolate the cabin from uncomfortable pressure influences caused by change of flight pressure altitudes when approaching limit difierential pressure or upon approach to the landing field. Failure on the part of the operator to accurately predict the ascent or descent pattern of the. aircraft in flight with this type of control imposes unreasonable discomfort on the passengers by greater rates of pressure change than necessary. The constant attention required if reasonable results are to be expected constitutes a costly annoyance and much more should reasonably be expected from a truly automatic control.

Systems have also been previously proposed which controlled cabin pressure in some predetermined relation to the change in pressure of the flight atmosphere so that the unreasonable burden of resetting the cabin pressure controls at critical times could be avoided. Such systems have failed to vary cabin absolute pressure for the occupants in terms of the very function upon which all ascents or descents are of necessity guided: an equal increment of flight altitude per unit of time. Instead such systems have proposed to vary cabin absolute pressure in proportion to changes in flight pressure and since air becomes rapidly more dense at low altitudes, the cabin pressure during ascents and descents change very rapidly at the low altitudes and considerably less rapid during ascents or descents at the higher altitudes.

The control system of the present invention,

obviates the difficulties had with previously proposed devices by providing means for controlling the absolute pressure within the cabin as a straight line function of the altitude of the aircraft, that is, altitude used in the standard aeronautical sense as meaning altitude based on absolute pressure in the standard international atmosphere and altitude as used herein is intended to mean pressure altitude.

The cabin pressure control effected by the control device of the present invention automatically produces without contingency or guesswork the slowest and therefore the most comfortable pressure change rate for the cabin occupants during descents and ascents generally encountered in aircraft flight operations. This is true because the factor controlling the change of cabin absolute pressure during either ascent or descent of the aircraft is the change of altitude of the aircraft. Since commercial aircraft have so-called placard or limit speeds, descents cant not b made faster than the rate which will produce the limit speed and it is conventional airline practice to descend in accordance with equal increments of altitude per unit of time. Similarly on the ascent the relatively constant available power from modern supercharged engines within the normal flight range makes it reasonable to use the excess power over that needed in overcoming level flight gravitational lift and drag forces to increase the aircraft altitude in equal increments, per unit of time. Thus the typical scheduled airline climb is one of constant flight speed and constant rate of altitude increase. And now since passenger pressure comfort in the cabin may be measured in terms of pressure change rates, it is clearly most desirable to control the absolute pressure within the cabin as a straight line function of the pressure altitude of the aircraft.

This invention also provides in a cabin pressure control system means for maintaining smooth automatic regulation of cabin pressure regardless of flow surges inadvertently imposed on the cabin ventilation system. These means actually anticipate the surges in the air delivered to the cabin and provide the necessary sensitivity to counteract and fully eliminate the adverse effects which would otherwise be highly detrimental to passenger comfort. Furthermore, cabin pressure schedule changes initiated by resetting the controls if and when desired may also impose flow variations at the discharge valve of the cabin. It is highly important that these surges be smoothly counteracted before they atfect cabin comfort, and that follow-up action to the pressure controls be provided to prevent hunting. This invention includes means to stabilize the control and give follow-up action with the special feature of not changing the normal cabin pressure control value in so doing.

To obviate the necessity of the operator of the pressure controls being forced to refer to special tables or charts, the present invention provides a control-setting apparatus on the cabin pressure control system which at all times will furnish to the operator a simple visual picture of the simulated cabin altitude which may be expected at any flight altitude during progress of the schedule and also shows the lowest cabin altitude which can be maintained within capability limits of the structure and supercharger apparatus at any given flight altitude for the particular aircraft. This control setting apparatus also constitutes a means of setting the control schedule for any flight long before the flight is started and to set the ratio between changes in absolute cabin pressure to changes in pressure altitude of the aircraft to the slowest cabin pressure change rate feasible during any given pressure altitude change.

The control system of the present invention also includes means for automatically controlling the time rate of pressure change for the cabin. This means, although ordinarily automatically operable, can be so set as to permit full manual control to be assumed in emergencies or during unusual flight procedures as may, for example, be undertaken during test flights or weather disturbances. The time rate of pressure change control means is normalLv operable in the preferred embodiment of this invention at predetermined limits to supervise and veto any action instigated by normal operation or resetting of the ratio-to-flight control means or the control action of the means for limiting either the cabin differential pressure or the ratio between cabin and flight pressures which may tend to make the cabin pressure change rate exceed the predetermined limits. Although the rate of pressure change means is normally an overriding control it may be set to assume primary charge of the rate of cabin pressure change when desired.

The control system of the present invention also includes a control means which will limit the maximum cabin diiferential pressure to values determined by the safety limits of the structure of the particular aircraft in which the control is incorporated even though the ratio-toflight or time rate of pressure change control means might tend to exceed these limits during flight operations at altitudes above levels initially expected.

To prevent overloading of the supercharger equipment or other air delivery means employed when flights are attempted at altitudes above those for which normal operation is intended, control means are also incorporated which will assume primary control of pressure within the cabin if a predetermined ratio of absolute pressure between the cabin and flight atmosphere tends to be exceeded and this control will limit the absolute pressure ratio to the predetermined maximum value.

Another object of the invention is to provide means to automatically depressurize the cabin at a comfortable pressure change rate in the case of an emergency landing at a fleld above that originally intended or in case of inadvertent or even intentional settings of the ratio-to-flight control means at pressure altitudes below the pressure altitude of the landing fleld. These means are also operable to obviate the pressurization of the cabin by such inadvertent settings while the aircraft is parked and so long as the aircraft is supported by the landing gear.

To prevent temperature of the air within the cabin from exceeding some predetermined temperature due to heating of the air by compression, temperature responsive means are incorporated in the system for automatically decreasing the pressure within the cabin when the temperature therein reaches the predetermined temperature. This thermo responsive means is sub- Ject to the time-rate-of-pressure-change control means in a similar manner to the means controlled by the landing gear thus preventing the pressure in the cabin from changing at a rate in excess of the rate imposed by the time-ratewfpressure-change control means.

Other features and advantages of the present invention will be apparent from the following description taken in connection with the accompanying drawings, in which:

Figure 1 is a diagrammatic view showing the cabin pressure control system as applied to a typical aircraft cab Figure 2 is a perspective view of the preferred embodiment of my ratio-to-flight pressure regulator with a portion of case broken away to more fully illustrate the same;

Figure 3 is a perspective view of the most imside view partly in section of the cause sure control relations between cabin pressure and flight pressure, the latter being represented as fiight altitude;

Figure 7 is a view of the dial and front face of my time rate of change cabin pressure regulator shown schematically in the system of Figure 1;

Figure 8 is a sectional view partly in elevation of my time rate of change cabin pressure regulator shown schematically in the system of Figure 1;

Figure 9 is a graphic plot of flight altitude versus time for typical control operations using time rate of change pressure control in direct response to the regulator elements shown in Figure 8; and

Figure 10 is a view similar to Figure 8 but showing my cabin pressure limits regulator shown schematically in the system of Figure 1.

The control system of the present invention, referring now to the drawing and particularly Figure 1 thereof, is shown as controlling the pressure within a sealed aircraft cabin 2|. Air is directed into the cabin through an air duct 22 arranged to deliver a flow of ventilation air to the cabin from a superchars'ing blower 22. The blower 23 is arranged to be supplied with air from a ram duct 24 and is driven through a shaft 25 by a speed controlled prime mover in such manner that a substantially constant rate of ventilation air flow is supplied through the duct 22 to the cabin 2|.

Although one cabin supercharging blower is shown for simplifying the illustration of the now preferred embodiment of the invention, it is obvious that dual blowers operating in parallel could be used. c

A check valve 21 is mounted within duct 22 and is so formed that it is normally opened by the flow of air through the duct, but which will close and seal the duct to maintain cabin pressure in the event of failure of airflow in the duct 22.

If desired, some conventional temperature regulating means may be mounted within the duct to control the temperature of the incoming air to maintain the air supplied to the cabin at a comfortable temperature. The air temperature conditioner 28 may also include its own automatic temperature control means operated in response to thermostats or similar controls disposed within the cabin.

A discharge duct 29 leading to an outlet 3| in the cabin wall is provided for the discharge of air from the cabin. The outlet II is preferably located on the cabin wall in a region where the pressure along the wall is due to surface air velocities slightly less than that of the ambient atmosphere. Interposed in or as a part of the duct 29 is a valve 32 which in its various operating positions varies the outlet area and thus provides any desired throttling of cabin air discharged.

Since a substantially constant rate of air fiow enters the cabin through the inlet duct 22, cabin pressure will be increased when the discharge valve 32 is closed or moved toward the closed position so that the air discharge is less than the air flowing into the cabin. On the other hand, if the valve is opened so that the air discharged is greater than the flow of incoming air, the pressure within the cabin will be decreased.

Although any means desired may be used to control the valve 32, in the illustrated embodiment of the invention the control of this valve is effected through an operating linkage 23, a

worm gear 24 carried by a shaft 35, and a prime mover 26, which in the embodiment illustrated is shown as an electric motor of the reversible split field series type. Although for the purpose of illustrating the invention, the'prime mover 26 has been shown as an electric motor, obviously hydraulic or pneumatic power could be substituted without departing from the scope of this invention. The linkage I2 is included in the valve drive in order to substantially relate the number of motor turns to change a given increment of cabin pressure over the full range of valve positions. The motor 26 may be energized either through a field coil circuit 21 or a field coil circuit as by power from some suitable source such as the battery II to open or close the valve 32, depending upon the direction of rotation of the motor.

A manually operable master switch 4| is interposed between the motor I! and the battery 39 and controls the motor circuit. Directional control of the motor, and consequently the valve 22 may be effected by selected operation of a control switch 42. In the embodiment of the invention illustrated, movement of the switch arm into engagement with the left-hand contact as viewed in Figure 1 closes the valve and engagement of the arm with the right-hand contact opens the valve.

To prevent over-travel of the valve, limit switches 43 and 44 are connected into the field circuits of the motor 3. The limit switch 43 is adapted to open the field coil circuit 38 when the valve 22 reaches its fully open position, while the switch 44 will open the field coil circuit 31 when the valve reaches its fully closed position.

Some direct manual control for the valve 32. although not shown, may be added or substituted for the master control switch if so desired.

With the directional control switch 42 in its neutral position, as shown in Figure 1, automatic control of cabin pressure is effected through a control relay 45. Relay 4! as shown for illustrative purposes is essentially a power amplifier in which very small currents from a battery 46 through two relay field coils 41 and 48 can be activated by the control circuits 49 and ii respectively, to bring about a fiow of relatively large currents in the motor field circuits 31 and 38 respectively. Energization of the control circuit 48 and its associated coil 41 will cause the armature 50 of the relay 45 to move into engagement with a contact 52 in the field coil circuit 31 to energize the same. Energization of the field coil 31 causes the motor 28 to close the. discharge valve 12 and similarly activation of the control circuit BI will cause the armature III to move into engagement with the contact 53 and result in opening movement of the discharge valve 32.

The relay 45 is provided with two centering springs 54 and 55 which not only hold the armature 50 in the open position shown in which neither control circuit is activated, but also moves the armature into its neutral position whenever both control circuits are energized. Although a single spring loaded relay element is shown, dual relay elements in series may be substituted if desired in the amplifier system to produce this circuit-cancelling action. Amplification of the discharge valve power by means of variable lowrange assistance, variable capacitance or variable inductance may also be substituted for the grounding type of control amplifier illustrated without departing from the scope of the invention.

It should be understood now that activation or grounding of the control circuit 49 will result in the discharge valve 32 moving toward its closed position to increase cabin pressure, and that grounding of the control circuit 9| will result in opening movement of the valve 32 to bring about a decrease in cabin pressure. Control circuit 99 may be activated by the action of either a ratio to flight cabin pressure regulator 99 or a time rate of change cabin pressure regulator 91. The control action of both regulators 99 and 91 is subject. however, as will be hereinafter more fully explained, to the overriding action of another primary regulator, a cabin pressure limits regulator indicated at 99 in Figure 1.

The control circuit 9| may be activated to call for decreased cabin pressure by any one of three primary regulators 99, 91 or 99, as well as a cabin overheating thermostat 99 or a landing gear switch 99. Specific transaction of each of these regulators and control elements will be discussed in turn.

The ratio to flight cabin pressure regulator 99 normally the most active of the primary regulators comprises, referring now to Fi ures 2 and 4, a sealed case 9| carrying a frame 92 on which is mounted an assembly of aneroid capsules 93 and an assembly of differential capsules 94. A control arm 99 is so pivotally mounted within the regulator that it is moved by the expansion or contraction of either or both the capsule assemblies 93 and 94. The one end of the arm 99 is interposed between a pressure increase control contact 99 and a pressure decrease control contact 91. The contact 99 is insulated from the case 9| and is connected to control circuit 49 by a lead 99. The contact 91 is also insulated from the case 9| and is connected to control circuit 9| by a lead 99. The control arm 99 is electrically grounded by means of a lead 19. The leads 99, 99 and 19 are connected to a suitable terminal socket II carried by the frame 92 and adapted to receive a conventional attachment cap when the instrument is mounted in the aircraft.

The control arm 99 is balanced between and separated from the contacts 99 and 91 by the capsule assemblies 93 and 94 whenever the pressure within the cabin corresponds to the control schedule of the regulator, 99. If cabin pressure, admitted through aperture I2, is substantially greater or less than scheduled, control arm 99 engages pressure decrease contact 91 or pressure increase contact 96 respectively, and control valve 32 is moved to a new position to bring about the predetermined scheduled cabin pressure.

The aneroid assembly 93 is of the conventional jacketed and sealed element type, and is rigidly mounted at its base to a slide I3 which in turn is adjustablw mounted on one leg of an intermediate slide member 1|. Adjustment of slide 13 on the intermediate slide H may be accomplished by means of a slide clamp 19 and an adjusting screw I9. The aneroid assembly 93 is flexibly supported on its expandable end by a resilient support 11 attached to the slide 13. The intermediate slide 19 is slidably mounted on the frame 62 and is held in abutment therewith by a slide bar assembly II. Adjustment of the position of the intermediate slide ll on frame 92 may be made by a lead screw I9. Lead screw II, threaded into the intermediate slide at one end as shown is supported and held in position coaxially'by lock nuts 99 straddling a frame pillar 9|. The lead screw carries a gear 92 at one end thereof and will be rotated when the gear 92 is rotated. The gear 92 engages a gear 93 on a dial setting shaft 94 also supported on the frame 92. The shaft 84 pierces the regulator case 9I through an air-tight seal and carries at the outer end thereof an adjusting knob 99.

Within the case "GI and also mounted on the shaft 94 is a drive gear 99 which drives through an idler gear a pinion 99 carried by the inner end of a short hollow shaft 99 loumaled on the frame 92. Fixed to the outer end of the shaft 99 by any means desired is a dial 99 carrying a suitable scale. The position of the dial 99 with respect to any adjusted position of the intermediate slide 14 may be made by adjustment of the position of the lock nuts 99. It is therefore possible to set the zero position of dial 9-9 without interfering in any way with the desired adjustment of contacts 99 or 91.

The differential pressure capsule assembly 94 is of the conventional thin walled corrugated disc type and is rigidly mounted at its base to a plate 9| and an adjustable support ring 92. The support ring 92 is in turn mounted on a capsule slide 93. The position of diflerential capsule assembly 9| with respect to the slide 93 is made adjustable by screws 9| and 99 carried by the slide 93. The position of the slide 93 with respect to frame 92 can be varied by means of a slidin clamp 99 and adjustment screw 91. The capsule assembly '99 is flexibly supported on its expandable end by a spring. support 99 attached to an integral finger of the base plate 9|. A tube 99 leads from a fitting I99 carried within an aperture formed in the rear of the case 9| to the interior of the capsule assembly 94. The fitting receives the one end of a tube I9I which leads to the exterior of the aircraft so that the interior of the capsule assembly 99 is in communication with the flight atmosphere.

The control arm 99 comprises an elongate .metallic member arranged to swing one end between the contacts 99 and 91 about either of the two independent but parallel hinge axes. The distance between these hinge axes'is made adjustable by a spreader screw I92. Static balance of the control arm is made possible by an adjustable counterweight I93. The primary support for the control arm 99 is provided at one of the hinge axes by jeweled bearings I94 carried by the free end of an arm of an L-shaped support I95.

The support I99 is hinged at the opposite end by a pin I99 mounted in lugs I91 formed integral with the frame 92. The other hinge axis of the control arm 95, as best seen in Figures 3 and 4, is established by a pin I99 carried by one end of a link I99, the opposite end of which is pivotally connected by a pin III to a lug II2 carried by the expandable end of the aneroid assembly 93.

From the structure thus far described it will be noted that an expansion of the aneroid assembly 93 would tend to rotate the control arm 95 toward the contact 99 while a contraction of the aneroid assembly tends to cause rotation or movement toward contact 91. If L-shaped support V I99 is held against movement, control as infinanced by the contacts is accordingly such as to maintain a constant value of pressure within the regulator case.

Associated with control arm support I99 there is provided a means for obviating any action on control arm 99 because of expansion orcontrac- 9 tion of the differential pressure capsule assembly 64, as well as for selecting any reasonable degree of action therefrom to combine with action from the capsule assembly 63,.

The components of the now preferred embodiment of this means comprises, as best seen in Figure 3, a ratio control link II3 pivotally attached at one extremity by a pin II4 to the L- shaped support I85 and at the other end to an elongate pin II5.- The opposite ends of the pin II5 are pivotally mounted in the arms of a yoke II8. A link 1 pivotally connected at one end by a pin I I8 to a lug II9 carried by the capsule assembly 64 is formed at the opposite end with a bifurcated member I28 which straddles the end of the link II3, the legs of the bifurcated member being formed with aligned openings for rotatably passing the pin I I5..

The free ends of the arms of the yoke II6 are connected by suitable pins I2I to the free ends of the arm of a U-shaped bracket I22, which in turn are connected adjacent the bow of the U to pivot pins I23 carried by the pillars 8| of the frame 62. The position of the U-shaped bracket may thus be adjusted about the axis of the pins I23, and this adjustment is preferably accomplished by means of a worm gear I24 carried by a shaft I25 and a sector gear I26 carried by the bracket I22 and mounted between the same and one of the pillars 8 I.

Fixed to the shaft I25, referring now to Figure 4, as a gear I21 which meshes with a second gear I28 carried by a shaft I28 supported on the frame 82. The shaft I29 pierces the regulator case 6| through an integral seal and carries at the outer end thereof an adjusting knob I3I. A gear I32 mounted on the shaft I28 engages an idler gear I33 meshing with a pinion I34 carried by a stub shaft I35. This shaft is coaxially mounted rel ative to the shaft 88 and carries at the outer end thereof a circular disc I36 having integrally formed therewith a pair of hands I31 and I38. As the hands I31 and I38 are integrally formed with the disc I36, the angle between them is fixed and can be predetermined. The cooperation between the hands I31 and I38 and the dial 88 will be hereinafter more fully explained.

It should be noted now that all of the pivotal axes of the component elements of the regulator 56 as defined by bearings I84, pins I06, I88, II4, III, H5, H8 and I23 are substantially parallel. The distance between centers of pins H4 and I I5 is made equivalent in length to the difference between centers of pins I2I and I23. Furthermore, the axes of pins I23 are coincident with the axis of the elongate pin II5 whenever the differential pressure capsule assembly 64 ,is subjected to zero pressure difference between the interior and exterior of the cabin. Since the separation'between the axes defined by the pins H4 and H5 is equivalent to that established by pins I2! and I 23, and adjusted position of the U-shaped bracket I22 may be chosen for which the hinge'center of axis of pins I2I is coincident with the axis of pin I I4. When the bracket I22 is so positioned the path of movement of pin II5 created by a contraction of the capsule assembly 64 is concentric with pin H4 and control arm support I85 is substantially locked against movement regardless of the differential pressure and control arm 65 is then subject only to absolute pressure.

Now, if the bracket I22 is moved to a position as shown in Figure 2 or 4, as soon as any contraction of pressure capsule 64 occurs such as would occur if the pressure in flight pressure tube 88 were allowed to become less than the pressure in the case 6|, then pin II5 tends to rotate around the new position of pin HI and the link I I3 will pull the support I toward the differential pressure capsule assembly, thus causing control arm 65 to rotate about pin I 08 toward engagement with contact 61. The farther the control arm support bracket I22 is moved from the position shown in Figure 3 the greater the action of the differential pressure capsule assembly on the control arm. This function of the adjustment of the bracket I22 will be better understood after an overall description of the regulatory action of the system has been made.

The time rate of change cabin pressure regulator 51, referring now to Figure 8, comprises a sealed case I42 in which is mounted a supporting frame I43. A slide I44 mounted on the frame I43 is movable thereon for adjustably positioning a differential pressure capsule I45. The capsule I45 is attached to the slide I44 by means of an adjustable support ring I46 and a base spool I41. An adjustable screw I48 permits adjustment of the slide I44 and the capsule with respect to the frame I43. An adjustment screw I49 permits the capsule to be vertically adjusted, as viewed in Figure 8.

The interior of the spool I41 communicates directly with the interior of the capsule I45 and through tube I5I to a fitting I52 mounted in an aperture I53 formed in the case I42.

A control arm I54 carrying at one end thereof a contact I55 is pivotally mounted adjacent the opposite end thereof to a finger I56 integrally formed with the frame I-43. To counterbalance the arm I54 a weight I51 having a tapped aperture therethrough is threadedly mounted to the one end of the arm. A link I58 pivotally interconnects control arm I54 and the expandable end of the capsule I45, the link being pivotally connected at one end to the arm and at the opposite end to a lug I59 carried by the capsule.

The control arm is electrically connected by a lead I6Ila to a circuit I68, the purpose of which will be hereinafter more fully explained. The control arm is so mounted within the instrument that the contact I55 is movable between a pressure increase contact I6I connected to control circuit 49 by a lead 49a and a pressure decrease contact I62 connected to control circuit 5I by a lead 5Ia. The contact I6I is mounted on an arm I63 formed with a threaded aperture receiving a threaded shaft I64 journaled in bearings I65 carried by the frame I43. Rotation of the shaft I64 and adjustment of the arm I63 and contact I6I is accomplished through a shaft I66 rotatably supported on the frame, and carrying at the inner end a gear I61 01' a bevel gear set, the other gear I68 of which is carried by the upper end of the shaft I64.

The shaft I68 carries at the outer end thereof a knurled knob I69 to permit the shaft I68 to be rotated to bring about the desired adjustment of the contact IN.

The contact I62 is carried by an arm I similar to the arm I63 and formed with a threaded aperture for receiving a rotatably mounted threaded shaft I12 journaled in bearings I13 carried by the frame of the instrument. The lower end of the shaft I12 carries a bevel gear I14 engaging and meshing with a gear I15 carried by a shaft I16 rotatably mounted on the frame of the instrument. The shaft I16 also carries at the outer end a knurled knob I11 to permit the shaft I16 to be easily rotated to bring about any selected adjustment of the contact I82.

The shaft I88 carries a gear I18 which,

through an idler gear I18, drives a pinion I8I. The pinion I8I is carried by a hollow shaft I82 which passes through a dial I88 carried by the frame of the instrument and supports at the outer end thereof a hand or needle I84 which serves as an index means for the scale of the dial I88.

It should now be seen that manipulation of the knob I88 adjusts the position of the contact "I and simultaneously sets the hand I84 to indicate the climb limit set by the adjusted position of the contact I8I.

The shaft I18 carries a gear I85 which engages and drives an intermediate gear I88 which in turn meshes with and drives a pinion I81 carried by the inner end of a stub shaft I88 coaxially extending through the hollow shaft. The shaft I88 carries at the outer end thereof a hand or needle I88 movable over the dial I88. As will be understood, rotation of the shaft I18 through manipulation of the knob I11 simultaneously adjusts the position of the hand I88 and the contact, I82 to provide a descent limit setting.

Insulated stops I8I are provided on inturned fingers of the arms I88 and HI to limit movement of the contacts I8I and I82 toward each other and in the now preferred embodiment of the invention the two contacts can be brought together to a position in which they are separated a distance slightly greater than the thickness of the contact I55 carried by the control arm I54.

A capillary tube I88 leads from the interior of the case I42 to the interior of the hollow spool I41 which is in direct communication with the interior of the difierential pressure capsule I45. Whenever pressure in the tube I88, spool I41, and pressure capsule I45 changes rapidly, a pressure diiference is built up across the capsule I45 due to the restriction to air flow in the small passage of the capillary tube I88, and the control arm I54 is subjected to large angular deflection. Similarly, very slow changes in pressure difi'erential across the capsule develops small angular deflections of the control arm I54.

In the illustrated embodiment of the present invention counterclockwise rotation of the control arm I54, as viewed in Figure 8, is brought about by decrease in the pressure of the air within the tube II and clockwise rotation of the arm is brought about by a pressure increase within the tube I5I. In accordance with conventional practice in aircraft rate of climb instruments, the capillary tube I88 may be of or filled with a material which increases its restriction to air flow in accordance with reduced air densities so that whenever so desired compensation may be obtained to permit relatively larger deflections of the control arm I54 for a given pressure difference at high altitudes. When so compensated a, given control arm deflection corresponds directly to a constant value of altitude change. Furthermore, temperature compensation may be provided as in present day aircraft rate of climb instruments by supplying an auxiliary gas filled diaphragm or a bi-metallic strip adjacent the pressure capsule I45.

The zero position of contact I8I is such that a very slight clearance is had between the same and the contact I55 when the control arm I54 is in zero pressure change position. A contact I84, carried at the free end of a spring arm I85 attached to a finger I88 integrally formed with the frame I48, is so held by the arm I85 relative to the contact I8l that for all positions of the latter from just beyond its zero adjustment position into the climb rate of control arm I54, engagement is maintained between contacts I8I and I84 by the resiliency of the spring arm I85. The contact I84 is electrically connected by a lead 88a to the circuit 88 through the resilient arm I85 and is thus electrically connected to control circuit 48.

However, upon adjustment of contact I8I to its zero position and into the descent range by proper manipulation of the knob I88 the engagement between contacts I8I and I84 is broken by a screw I81 adjustably mounted on a lug I88 integrally formed with the finger I88 and extending substantially normal thereto.

The zero position of contact I82 is such that a very slight clearance is had between the contact I55 when the control arm I54 is in the zero pressure change position. A contact I88, connected by leads 88a and 88 to circuit 5|, is carried at the free end of a resilient finger 28I having the opposite end anchored to an arm 282 integrally formed with the frame I48 and projecting upwardly therefrom as viewed in Figure 8. The resilient finger 28I is so positioned that for all positions of contact I82 from just beyond its zero adjustment position into the descent range of control arm I54 engagement is maintained between contacts I82 and I88. Upon adjustment of the contact I82 to its zero position or anywhere into the climb range this engagement is broken by the screw 288 adiustably mounted in a lug 284 extending substantially normal from the finger 282.

The cabin pressure limits regulator 58, referring now to Figure 10, comprises a sealed case 288 of insulating material in which is formed a tapped aperture for receiving one end of a tube or conduit 288a leading to the tube IN. The case houses a suitable frame 285 on which is mounted a hollow spool 288. A diiierential pressure capsule assembly 281 is rigidly secured to the one end of the spool 288 and the interior of the latter directly communicates with the interior of the capsule assembly. A tube 288 leads from a fitting 288 in communication with conduit 288a to the interior of the spool 288 to the bly 281.

end that the interior of the capsule is subjected to flight pressure. A control arm 2I8 is pivotally mounted adjacent one end thereof to an L- shaped support 2 suitably supported by the frame 285. To counterbalance the arm 2I8 a weight 2I2 is threadedly mounted to the one end of the arm for movement longitudinally of the same.

The control arm 2 I8 and the capsule assembly 281 are pivotally interconnected through a short link 2I8 the opposite ends of which are connected respectively to the capsule assembly and the control arm 2I8 through suitable pivot pins.

The position of control arm 2I8 is thus varied by expansion or contraction of the capsule assem- The free end of the arm 2I8 is thus movable between a pair of contacts 2I4 and 2I5 carried by rods 2I8 and 2I1 respectively, adjustably carried by fingers 2I8 and 2I8 respectively, supported on the frame 285, and insulated therefrom. A bumper 228 formed by an extension of finger 2I8 limits the angular deflection of the control arm 2I8. The position of the contacts 2I4 and 2I5 can be selectively adjusted by rotation of the rods 2I8 and 2" through engagement of the tool receiving members carried at the one end of each of the rods.

The leads 22I, 222, and 228 respectively, electrically connected to contacts 2I5 and 2I4 and the control arm 2I8, respectively are attached to 13 a suitable lead-in receptacle 224 which takes an attachment cap (not shown) carrying contacts connected to circuits I50, 5| and a suitable grounding connection respectively.

Control arm 210 is held in engagement with ground circuit contact 2I5 whenever the pressure difierential acting on differential pressure capsule assembly 201 is less than a predetermined value. If the acting pressure differential is greater than this limit value, control arm 210 first disengages from contact 215 opening the control circuits of regulators 55 and 51 and upon further increase in differential pressure moves into engagement with contact 214 which results in energization of control circuit 5| and therefore opening movement of the control valve 32 until the limit cabin pressure is no longer exceeded.

Cabin pressure limits regulator 58 is arranged furthermore to decrease the limiting differential pressure in accordance with a fixed limiting ratio of the absolute pressures existing across the pressure capsule assembly 201. This limiting ratio function is derived from the action of an aneroid 225 on diflerential pressure capsule 201. The aneroid capsule 225 is supported by a ring 221 carried by a slide 220 mounted on an enlarged portion of the frame 205. An adjustment screw 229 carried by an upstanding boss formed integral with'the enlarged portion of the frame 205 permits the slide 228 and the aneroid 225 to be adjusted longitudinally of the frame 205. A second adjustment screw 23l permits axial adjustment of the fixed side of the aneroid 225 with respect to the slide 220. A post 232 carried by the expandable side of the aneroid capsule 225 is adapted to engage against the underside of the control arm 2I0 to augment the action of capsule 201 on the control arm whenever aneroid 225 expands more than a predetermined extent as a result of the reduction of cabin absolute pressure below a predetermined value. By adjustment of screws 229 and 231, the position of the aneroid can be selected which will bring about the desired cooperation between the same and the capsule assembly 201 to maintain a balance of control arm 210 between contacts 2 and 2I5 in accordance with a preselected ratio of the absolute cabin pressure to absolute flight pressure, that is a preselected cabin compression ratio whenever regulators 56 and 51 signal for control of the valve 32, which would result in a cabin compression ratio in excess of the value preselected. It will be seen that as long as control arm 210 is disposed between contacts 214 and 215, the cabin compression ratio will remain constant, but should the preselected compression ratio be exceeded, the action of capsules 201 and 225 will be sufficient to move the control arm 2 I against contact 2I4, resulting in energization of control circuit 5| and opening movement of the valve to reduce absolute pressure within the cabin and consequently the cabin compression ratio.

Two associated control elements, namely, the cabin overheat thermostat 59 and the landing gear switch 50 are included in the cabin pressure control system as shown in Figure 1. Each of these elements is connected to a special valve opening control circuit 233, which when grounded energizes a ratio regulator cut-out relay 234. The relay 234 comprises, referring now to Figure l, a field coil 235, an armature 235, a spring 231, and a pair of contacts 230 and 239. Contact 238 is electrically connected to ratio regulator ground lead 1| while contact 239 is connected to the control circuit 5|. The armature carries a contact which is connected to the intermediate ground lead I50. The spring 231 normally maintains the contact carried by the armature in engagement with the contact 238 so that the regulator 56 is normally operative. However, when control circuit 233 is grounded as by the overheat thermostat 59 or the landing gear switch 50, the coil 235 of the relay 234 is energized and the contact of the armature 238 is moved into engagement between the contact carried by the armature 235 and the contact 238. The disengagement of these contacts breaks the grounding circuit for regulator 56 and energizes the valve opening control circuit 5|, which, through the valve mechanism hereinbefore described, causes the valve to open, thus bringing about a reduction in cabin pressure or a decrease in the absolute value of pressure in the cabin. As may be noted from the circuit diagram of Figure 1, however, this valve opening action is still supervised by regulator 51 to limit the rate of pressure decrease in the cabin.

The overheat thermostat 53 mounted within the cabin and electrically grounded as shown in Figure 1 comprises a bi-metallic strip 24I carrying a contact 242 engageable with a contact 243. The strip is so formed that it will move contact 242 toward contact 243 upon increase of cabin temperature. Contact 243 is made adjustable in position by the screw 244 and is connected to control circuit 233 through a manually operable switch 245. By setting contact 243 to engage contact 242 at a. particular upper tolerance limit of cabin temperature and with switch 245 closed, cabin differential pressure may be limited to a value which will prevent serious cabin temperature discomfort in favor of cabin pressure comfort, even though unreasonable schedules of cabin pressure control are attempted during extremely warm weather. A reduction of cabin temperature is normally associated with a reduction of cabin pressure since incoming air is heated by the supercharger somewhat in proportion to the cabin differential pressure. For most high altitude operations in normal or cold weather additional heat will be required to maintain a 70 F. temperature level even while flying with full cabin pressurization. This additional heat will be supplied by some form of heater, not shown, but associated with the air temperature conditioner 23 shown in Figure 1.

For extreme warm weather operations with no cooling facilities for the incoming air, cabin temperature may become excessive upon attempting extreme pressurization and then cabin overheat thermostat 53 may be made operative to automatically try to reduce cabin pressure to lower the cabin temperature below the set limit if ever this set limit is exceeded. With intercoolers and an expansion turbine refrigeration apparatus installed in air temperature conditioner 23 to cool the cabin to comfortable temperature during extremely warm weather, cabin overheat thermostat 53- automatically tries to lower the cabin pressure and, as a consequence, make available more of the supercharger power for refrigeration if ever the cabin temperature exceeds the set upper limit. The overheat thermostat may, for example, be set to or Fahrenheit.

Landing gear switch 50 is mounted on the landing gear of the aircraft and is electrically grounded as shown in Figure 1. The landing gear shown is of the conventional retractable design in current use on most all present day.

commercial aircraft. A hydraulic pneumatic shock strut 245 is movable in a cylinder housing 241 in such manner that the strut and a wheel 248 attached to this strut move upward under load as upon application of the aircraft weight against the ground in landing. In landing position the strut and wheel move downwardly with respect to the aircraft when the aircraft weight is lifted oil the gear as occurs when the aircraft becomes airborne. The strut 245 is provided with a contact 249 which is electrically grounded. Cylinder housing 241 carries a contact 25I which is so spaced from contact 249 as to engage it only when weight of the aircraft is on the landing gear. A normally closed manually operated switch 252 permits the landing gear switch to be removed from the system ii desired, as when testing the system with the aircraft on the ground.

Inadvertent settings 01' regulators 55 or 51 to cabin altitudes below those of the level of the landing field are thus prevented from either causing cabin pressurization prior to flight, even though the system is otherwise made operative, or from maintaining cabin pressure once the aircraft lands.

Cabin pressure apertures 12, I52 and 258a of regulators 55, 51 and 58 respectively, are communicated to cabin pressure through an anticipator system in order to render these control elements extremely sensitive to transient changes in pressure during cabin pressure operation. By this system trends of cabin pressure and potential surges in cabin pressure are made reactive upon the regulators prior to any appreciable change in the cabin pressure. The anticipator system per se forms no part of the present invention and is disclosed and claimed in United States Patents Nos. 2,407,257 and 2,407,258 issued from my copending applications, Serial Numbers 429,901 and 446,039. For this reason, only those details of the anticipator system as are necessary to a full understanding of the present invention will be described herein.

This system, referring now to Figure 1, comprises a low capacity air flow circuit arranged in parallel with the cabin ventilation circuit. In the anticipator circuit air enters the Pitot tube 253 pointed upstream in duct 22, passes through a conduit 254, and discharges into a venturi 255 located in the upper duct 29. The pressure in Pitot tube 253 is slightly higher than cabin pressure by the amount of ram and pressure drop from this position in duct 22 to the cabin. The pressure in venturi 255 is slightly less than cabin pressure by the amount of Venturi suction and pressure drop to this position in duct 29 from the cabin. A small casing 255 is interconnected into the conduit 254 at a substantially mid-point therein. The pressure within the casing 255 is substantially equivalent to cabin pressure whenever stabilized and equivalent air flows are passing into and out of the cabin. Conduits 251 and 258 leading from regulators 55 and 58 respectively. are connected into the casing 255 so that the interiors of the instruments are subject to pressure variations in the anticipator system. At a point in conduit 254 adjacent the venturi 255 wherein the pressure is less than cabin pressure and very nearly equal to the pressure venturi 255, a conduit 259 leading to the pressure responsive element of regulator 51 is connected into the conduit 254.

The cases of either or both the regulators 55 and 58 as well as the pressure responsive ele- 16 ment of regulator 51 may be made directly responsive to the cabin pressure rather than to the pressure in the anticipator system, if at any time it is so desired, by suitable operation of plug valves 25I, 252 and 253, respectively.

An adjustable restrictor valve 254 is provided in conduit 254 in order that the non-transient or normal static pressure at the medial position in conduit 254 may be readjusted manually to equal cabin pressure even though minor variations are more permanently made in the effective pressure drop through duct 22 downstream of the Pitot 259 as may be effected by ventilation distribution adjustment, or upon any minor change in the pressure drop in duct 29 upstream of venturi 255 as may be effected by changes in leakage through the seams of the cabin. A relatively small structural seam leakage is to be expected and it is further to be expected that this small leakage will increase somewhat during the life of the aircraft.

The regulators 55 and 58 are connected into the anticipator system at the substantially neutral midway point as hereinbeiore brought out because these regulators require close control of an absolute cabin pressure and are preferably made sensitive to that exact pressure at the same time obtaining equal response to changes in cabin in-flow and out-flow. However, regulator 51 is preferably connected to the anticipator circuit at a point closely adJacent to the venturi 255 in order that an even greater speed of response is attained with respect to change in cabin outflow as controlled by valve 32 than is attained with respect to change in the flow of air into the cabin.

As a safety feature for protection of the structure of the airplane cabin in case of emergency, a relief valve 255 is furnished on the cabin wall as shown in Figure l. The setting 01 this simple spring-loaded valve is adjusted by means of adjustment nut 251 to a value above that limited by pressure limits regulator 58. A handle 258 is provided to permit normal operation of the valve to keep it free and to check it for freedom. A lead screw 259 is mounted adJacent the handle to hold the valve open if so desired in emergency.

Ratio to flight cabin pressure regulator 55 presents a dial and hands arrangement to the operator as shown in Figure 5. The dial 88 is arranged for rotation concentric with the disc I35 carrying the hands I31 and I38. Numbers representing standard pressure altitudes are so marked on the dial 98 that the difference between the standard pressures corresponding to any two altitude readings indicated thereon by the hands I31 and I38 is exactly equal to the predetermined limiting diilerential pressure for the airplane cabin at the altitude indicated. Hand I31 is labeled as showing ratio limit flight altitude and hand I38 is labeled as showing ratio limit cabin altitude. A fixed indicator 2" is mounted at the top of the face to mark a pressurizing'altitude setting as read on dial 98. Some desired index marks such as shown at 212 on indicator HI and hand I38 can be used to indicate the range of cabin pressure change which may be expected while flight is varied between altitudes indicated by the index marks 213 carried by the indicator 2" and hand I31.

Time rate of change cabin pressure regulator 51 presents a dial and hands arrangement to the operator as shown in Figure '1. Pressure change rate setting for limiting cabin climb is sassc 17 indicated by needle I54 over the markings on dial ll: while pressurechange rate setting for limiting cabin descent is indicated by needle I85.

Control ,of cabin pressure as regulated by my cabin pressure control system herein described is intended to .always be limited by regulator 55 within a maximum predetermined differential pressure which is the safe maximum for the cabin structure, and within a maximum predetermined compression ratiowhich is the safe and attainable maximum for the cabin pressure air supply superchargers. Typical values for such limits are 8.5 inches of mercury differential pressure and 1.75 compression ratio. This differential pressure represents an 8000 foot eifective cabin altitude at 20,000 feet flight pressure altitude and the 1.75 compression ratio represents an effective 11,300 foot altitude at 25,000 feet flight pressure altitude.

There is shown in Figure 6 graphic illustrations of pressures which are limited by regulator 55 as well as the control functions of ratio to flight cabin pressure regulator 55. The pressure of the standard atmosphere as a function of flight altitude is indicated by the curve 215 and so labeled. Flight altitude is scaled in thousands of feet along the abscissa and pressure is sealed in inches of mercury absolute along the ordinate of the graph. The limit cabin differential pressure of 8.5 p. s. i. with respect to the atmospheric pressure is shown by the curve 215. Note that the vertical or ordinate distance between this curve and the atmospheric pressure curve is a constant fixed quantity for all flight altitudes. The limit cabin compression ratio of 1.75 is shown by the curve 211 and is so labeled. It should be noted that the total ordinate of this curve is a fixed multiple of the ordinate for the atmospheric pressure curve of any flight altitude. Also note.

that the limit curves so chosen in this example case intersect at an altitude of 25,000 feet. Below 25,000 feet maximum differential pressure is made the limit condition but above 25,000 feet the compression ratio becomes the limitation. To produce this control limitation, post 232 on the expandable side of aneroid 225 in pressure limits regulator 55 as shown in Figure must, for the above control settings, remain disengaged from control arm 2 II until 25,000 feet is attained and must become engaged at all altitudes above 25,000 feet with such force as to reduce the differential pressure acting on differential pressure capsule 201 in accordance with the pressure indicated by the difference in ordlnatesof the compression ratio limit curve and the atmospheric pressure curve 215 of Figure 6.

Connecting the atmospheric pressure curve 215 and the cabin differential pressure curve 215 in Figure 6 there is a horizontal pressure control curve A. This curve intersects the atmospheric curve at 2,000 feet altitude and intersects the cabin differential pressure curve at 11,600 feet altitude. Control curve A represents a schedule of cabin pressure during transition of night altitude from 2,000 feet to 11,600 feet and for this schedule cabin pressure is seen to remain constant at a value of 27.82 inches of mercury absolute during flight between these altitudes. Curve A-can furthermore be considered as one component of an overall cabin pressure schedule for aircraft flights from below 2,000 feet to well above 30,000 feet. For such a schedule, cabin pressure would remain equal to atmospheric pressure during flight up to 2,000 feet as represented by the atmospheric pressure curve 215. During flight between 2,000 feet and 11,600 feet, the

cabin altitude would remain substantially constant at 2,000 feet. as represented by curve A. For flight from 11,600 feet to 25,000 feet the cabin differential pressure would remain constant at 8.5 inchesof mercury as represented by the differential pressure curve 216.

Above 25,000 feet the cabin pressure would be controlled at a fixed ratio of 1.75 times the pressure of the ambient atmosphere as represented by the compression ratio limit curve 211. This control schedule can be accomplished with the cabin pressure control system shown in Figure 1, first by setting the dial 50 of ratio to flight cabin pressure regulator 55 to a pressurizing altitude of 2,000 feet. this setting being shown in Figure 5. Next, time, rate of change cabin pressure regulator 51 is to be set to reasonable rate comfort limits, for example, with hand I84 at 600 feet per minute climb and with hand I89 at 400 feet per minute descent as shown in Figure 7. With switches 4I and 42 in closed positions asshown in Figure l, the above schedule is now operative. The bracket I22 in regulator 56 will for this schedule line up in the horizontal zero position shown for it in Figure 3. For flight altitudes below 2,000 feet, aneroid capsule 63 in regulator 55 will remain sufliciently collapsed to hold the control arm 55 against contact 51, thereby closing valve opening circuit H which results in control valve 32 being open but stationary as a result of the open position of limit switch 43. As the pressurizing altitude of 2,000 feet is reached, aneroid capsule 53 has expanded sufliciently to balance control arm 55 between contacts 66 and 61. As further ascent is made, aneroid capsule 63 expands. engaging control arm 55 with contact 55, thereby energizing valve closing circuit 45 to the end that the valve 32 moves toward a closed position such that cabin absolute pressure is increased sufiiciently to collapse the aneroid capsule back to a position of contact balance. A 2,000 foot cabin altitude is thus maintained.

The scheduled pressure limits are maintained above 11,600 feet flight altitude by regulator 58 as described heretofore. If a 600 foot per minute rate of cabin climb is not exceeded during ascent, and if a 400 foot per minute rate of cabin descent is not exceeded during descent, regulator 51 will remain inoperative in the system. If these rate limits are exceeded, rate of pressure change regulator 51 will, by means of control arm I54, actuate valve closing circuit 49 in opposition to valve opening circuit 5| or vice versa, thereby opening the control of motor 35 from relay 45 and stopping all action on valve 32 until cabin pressure change rates stabilize within these limits.

It will be seen, referring again to Figure 6, that a number of schedules possible with the system of the present invention in addition to that represented by curve A, have been plotted thereon.

Control of cabin pressure in accordance with a schedule along any of sloping curves such as control curves B C D, E, F can be represented by a fractional ratio. An ordinary ratio to flight control would be represented by the following ratio:

where P is the flight absolute pressure at the pressure altitude at which pressurization of the cabin is to begin, and k1 is a predetermined constant which for example may have a useful range from 0 to .6. In the latter case this would be equivalent to saying that the decrease in cabin 19 pressure above a predetermined pressurizing altitude will be .6 times the decrease in flight altitude pressure above this same predetermined point. If regulator 58 were to consist of two coactins aneroid units, one sensitive to cabin pressure and one sensitive to flight pressure, then the above expression for the ratio control would be clearly applicable. However, since regulator 56 comprises an aneroid sensitive to cabin pressure and a differential pressure capsule exposed to the difierence between cabin pressure and flight pressure, the above expression may be more clearly applied in the form:

(P)- (cabin absolute pressure) (cabin absolute pressure) (flight absolute pressure) where P is the flight absolute pressure at the pressure altitude at which pressurization of the cabin is to begin, and k: is a predetermined constant which for example may have a useful range from to 1.5. The expression for in and ka define identical schedules of cabin pressure in that they each represent a straight line when plotted on a graph of cabin pressure as one ordinate and flight pressure as the other ordinate. In Fig. 6, the form of cabin pressure control in which the change of cabin pressure is a direct ratio to the change of flight pressure is represented by the sloping curved dotted lines. The sloping curves, although straight when plotted on a graph of cabin pressure as-one ordinate and flight pressure as the other ordinate are not straight when plotted as in Figure 6 but are bent with a greater slope at lower altitudes than at higher altitudes due to the form of the coordinate plot of the abscissa and ordinate in Figure 6. The solid straight curves in Figure 6 however represent ratio to flight pressure control in which the change of cabin pressure is a direct ratio to the change in flight altitude and not flight pressure. This control is a considerable improvement over the form of the control represented by sloping curves since during the operation of aircraft climbs and descents are normally gauged by instruments which read in terms of altitude. Thus, for the greatest cabin comfort to passengers, the change of cabin pressure should be controlled at a minimum rate in relation to the pressure altitude variation of the aircraft and not its flight pressure variation. This type of control can be best defined by its straight line relationship in Figure 6 but can also be expressed:

(P) (cabin absolute pressure) =1 (flight altitude) (pressurizing altitude) where P is the flight absolute pressure at the pressure altitude at which pressurization of the cabin is to begin, and ka is a predetermined constant. If pressures are expressed in inches of mercury and altitudes are expressed in thousands of feet the useful range of this ratio k: is from 0 to about .00007.

The equations for k1 and k: represent a ratio to flight pressure control which can be defined as a straight line on a graphic plot of cabin pressure as a function of flight pressure. The equation for k: represents a ratio to flight pressure control which can be defined as a straight line on a graphic plot of cabin pressure as a function of flight altitudes. Two other forms of ratio control are similar to the two previously mentioned but as should now be understood are far less desirable. These are, first. one which plots a straight line for cabin altitude varying as a 20 function of flight altitude, and secondly, one which plots a straight line for cabin altitude varying as a function of flight pressure.

The straight curves are then preferred control schedules to that shown by the sloping curves. However, since the fractional expression for the ratio k1 in the sloping curves is simple to express, this general ratio notation will be used for the ratio to altitude curves. For example, as shown by the table in Figure 6. the simplified approximate valued the ratio for curve B is V4 or .25. Its exact value would have to be expressed by ks.

Control curve B represents a schedule of cabin pressure during transition of flight altitude from 2,000 feet to 15,500 feet and for this schedule, cabin pressure is seen to vary from a value of 27.82 inches of mercury absolute (2,000 feet altitude) to a value of 24.98 inches of mercury absolute (4,900 feet altitude) during flight from 2,000 feet to 15,500 feet.

This cabin pressure control schedule can be accomplished with the cabin pressure control system shown in Figure l, first by setting the dial SI of ratio to flight cabin pressure regulator 58 to 2,000 feet. Next the ratio limit flight altitude hand I31 is to be set to 15,500 feet. The ratio limit cabin altitude hand I" will then automatically read 4,900 feet. The time rate of change cabin pressure regulator 51 may be set as in the previous example to limits similar to those shown in Figure '7. This control schedule including curve B is now operative.

Other typical ratio control schedules possible with the system of the present invention are indicated by control curves C, D, E, and F in Figure 6. The curves show the desired straight line form of control as plotted thereon For curves A and C the pressurizing altitudes are 2,000 feet, the ratio limit flight altitudes are 11,600 and 25,200 feet, respectively, and the net or average control ratios are 0 and V2, respectively. For curves D, E and F the pressurizing altitude is 6,000 feet, the ratio limit flight altitude's are 17,300, 22,000, and 28,500, respectively, and the net or average control ratios are again 0. and V2, respectively. The approximate average ratios are marked around the outer rim of the face of regulator 58 as shown in Figure 5. An arrow 219 on the disc I" indicates the average ratio reading as determined by adjustment of the hand III of the regulator.

For control along curves A or D of Figure 6, the position of the U-shaped bracket I22 and regulator 50 is moved through adjustment of the knob 18 to a position substantially parallel to control link II! as best seen in Figure 3. For control along curves B or E an intermediate angular position of the bracket I22 approximately as shown in Figure 4 is to be used. For control along curves 0 or F a larger angle of this bracket with respect to control link H2 is to be used.

Action of aneroid assembly 03 and differential pressure capsule assembly 84 is directly related to pressure changes, not altitude functions, so that when regulator 50 is set for ratio control, the ratio schedules would normally be in accordance with the ratio curves shown in broken lines and identified in Figure 6 as B C E and F1. The slope of each of the curves Bl Cl, E1 an F1 is greater than the slope of the straight line curves B. C. E and F in the low altitude range of each curve where cabin differential pressure is low, and the slope is greater aasae in the higher altitude range where higher cabin differential pressures exist. Regulator 50. however, includes means for so adjusting the control arm linkage from the differential pressure capsule that action of the ratio producing or slope producing elements is decreased at low differential pressures, and is increased at higher differential pressures. Thus the ratio control curves shown in broken lines are in effect bent to the straight line slope of the curves B, C, E and F.

This adjustment is made by moving differential pressure capsule assembly ll to the right as viewed in Figure 4 along the slide 03 to a position in which link 1 subtends a substantial angle from the vertical. If the link H1 is moved to a position in which the angle is within the range of 35 to 50 and the length of the link I" is relatively short, the necessary geometric arrangement is had to modify the ratio control curves to the solid curves desired. It will be seen that diflerential pressure capsule assembly 04 contracts substantially vertically under the infiuence of increasing difi'erential pressure. During its contraction in the low differential pressure range, it will cause pin Hi to rotate about pins III at a relatively slow rate due to the large angle between link 1 and the direction of capsule movement. Howevenas higher differential pressure is reached, link 1 moves toward the vertical or in the path of capsule motion and a greater rate of rotation f.pin I about pins III is obtained thereby increasing the slope of the ratio control curve as desired.

Thus, for example, for the schedule represented by the curve B, as 2,000 feet ambient altitude is reached, aneroid capsule 03 will have expanded sufliciently to balance control arm 65 between contacts 65 and 51. As further ascent is made, aneroid capsule 63 expands and moves control arm 65 into engagement with contact 60 to close circuit 49, which as should now be understood results in closing movement of valve 12 to a position such that a differential of cabin pressure above ambient pressure is built up, and since cabin absolute pressure is thereby increased, the aneroid 63 starts to collapse back to an equilibrium position. Now, however, the contraction of capsule 64, under the influence of the differential pressure acting thereon, rotates the yoke H0 counter-clockwise about pins III, pulls control arm support and control link Hi to the right and moves control arm 05 toward contact 51. As the support bracket I22 in this schedule will have been adjusted to a position approximately as shown in Figure 4, the combined action of aneroid capsule G3 and differential capsule 64 on control arm 65 is such as to balance it between contacts thereby satisfying the control curve B.

It may be noted in Figure 6 that control curve F connects the atmospheric pressure curve to the compression ratio limit curve without limitation or intersection by the differential pressure limit curve. This simply means that the compression ratio limit is reached before reaching the maximum differential pressure limit. The ratio limit flight altitude reading furnished by the hand I31 of the regulator 56 still indicates the correct limit reading, however, since the peripheral altitude markings on dial 90 are spaced so that a fixed angle represents a fixed pressure difference in the range from 0 to the compression ratio limit intersection (which is 25,000 feet in the example shown) and are spaced so that a fixed angle schedules time rate of pressure change regulator.

51 was used only as a veto device to prevent the occurrence of uncomfortable rates of pressure change during resetting of ratio to flight regulator 58 on the link. -Now another type of control can be accomplished with this control system, namely, time rate of pressure change control in which operation regulator 51 becomes the primary controller, and regulator 56 merely stops the controller cabin climb on descent at a set altitude. Pressure limits regulator 58 always maintains control over the extreme limit pressures.

Referring now to Figure 9. there are plotted some typical flight and cabin pressure schedules during the operation of time rate of pressure change control. The ordinate of this graph is scaled in thousands of feet of altitude and the abscissa is scaled in minutes of flight duration. Curve OABCDEFGHZ represents a typical flight starting at sea level, point 0, climbing to 20,000 feet at the rate of 1,000 feet per minute, maintaining level flight for 80 minutes, descending to 18,000 feet at a rate of 500 feet per minute, maintaining level flight for 8 minutes, climbing to 22,000 feet at 500 feet per minute, maintaining level flight for 68 minutes, and finally descending to airport Z at 4,000 feet at the rate of 1,000 feet per minute. The lower limit of cabin altitude dictated during this fiight by the example value of 8.5 inches of mercury peak cabin diflerential pressure is shown by the curve JKLMNOPQRS. Now by setting both hands I84 and I8! of regulator 51 to the 500 feet per minute climb mark. a cabin climb along a curve OT may be accomplished. To prevent the cabin from continuing to climb right on up to the flight altitude it may be stopped at the 9,000 foot level indicated by curve TU by setting regulator 56 to zero ratio and to 9,000 feet pressurizin altitude. The connection of valve opening circuit 49 to regulator 56 is automatically broken by the separation of contacts I52 and IS! in regulator 51 when both indicators are set to a climb value. The valve closing circuit from regulator 56 is still active, however, so that the 9,000 limit altitude will be held after the cabin pressure has been reduced to the corresponding value. If a 300 feet per minute climb setting for both hands of regulator 51 had been chosen the climb schedule would be in accordance with curve OB KC the set rate of climb being interrupted from B to K by the action of pressure limits regulator 58.

Curve TU once obtained in the cabin pressure control schedule may be maintained by the setting above or may be maintained by two alternate procedures, the first of which is to set the time rate of pressure change hands to zero, or secondly to set hand I04 anywhere in the climb range and hand I89 anywhere in the descent range thereby allowing regulator 55 to maintain the 9,000 feet cabin altitude. At point U on this curve. pressure limits regulator 58 would take over continuing through UOPV.

A time rate of pressure change control of descent may be started at apoint W or X, for example. on curve VX. If both hands of regulator 51 are set to 250 feet per minute descent at point W and if regulator 55 is set to an equalizing altitude of 4,000 feet or lower, the cabin will descend in accordance with curve WA If both hands of regulator 51 are set to 400 feet per minute descent at point X, and if regulator 56 is set to a pressurizing altitude of 4,000 feet and to zero ratio, then the cabin will descend in accordance with curves KY and Y2. The altitude limit action of regulator 58 is accomplished as a result of automatic separation of contacts lil and I in regulator 51 at the time that hand Ill is set into the descent range, so that only the valve opening circuit ii is active during the descent.

Operation of the control system in Figure l on a time rate of pressure change schedule is generally less desirable than operation on a ratio to flight schedule. The reasons are obvious for, as

' ing a predetermined pressure altitude for conpointed out hereinbefore, several different settings of time rate of pressure change regulator must be made at specific times during the flight. This is often inconvenient and a burden to the flight engineer or other operator. The continuous setting and resetting at times may cause unnecessary discomfort to the passengers and the failure to remember to start the cabin down at the correct time unnecessarily complicates the change of cabin pressure. For example, in Figure 9, descent of the cabin cannot be started much before point W or the differential pressure limit will interrupt it. If the start of cabin descent is delayed 6 or 8 minutes, then a much more abrupt descent must be made or else-the cabin will become equalized with flight pressure during the descent and a higher rate of descent will automatically be imposed.

It is furthermore difficult to ascertain the ex-.

act time that the airplane's flight will reach a predetermined altitude. On the other hand, with ratio to flight control the regulator settings may very often never have to be changed throughout the flight and in cases where airports of departure and destination difler greatly in altitude, regulator settings can be changed to meet the most comfortable schedule any time prior to the start of'descent. Descent of the cabin in ratio to flight control automatically starts when the airplane begins to descend. However, there are times when a fast descent of limited extent may be necessary through an opening in the overcast. For this and other similar occasional conditions it is important and desirable that the time rate of pressure change control be readily available with convenient means of making it' operative over the ratio to flight type of controli Although the now preferred embodiment of the invention has been shown and described herein, it is to be understood that the invention is not to be limited thereto for the invention is susceptible to changes in form and detail within the scope of are appended claims.

I claim:

1. An aircraft compartment pressure control comprising: means for delivering air to said compartment under a pressure greater than ambient flight pressure; means for discharging air from said compartment; means for varying the rate of air discharge from said compartment relative to the rate of air delivery to said compartment whereby the absolute pressure within said compartment may be varied; means, including a first capsule subject to cabin absolute pressure and a second capsule subject to cabin differential prestrolling said last-named means for regulating the absolute pressure in said compartment in such a manner as to change said pressure in inverse straight-line proportion to changes in pressure altitude of said aircraft above said preselected pressure altitude; and means for preselecting the pressure altitude at which said controlling means is made operative; said preselecting means being changeable during flight of said aircraft and 0perable' independently of said control means whereby said pressure altitude at which said control means is made operative may be altered during flight of said aircraft without altering the said proportion.

2. An aircraft compartment pressure control comprising: means for delivering air to said compartment under a pressure greater than ambient flight pressure; means for discharging air from said compartment; means for varying the rate of air discharge from said compartment relative to the rate of air delivery to said compartment whereby the absolute pressure within said compartment may be varied; control means, including a first capsule subject to cabin absolute pressure and a second capsule subject to cabin differential pressure coacting through an interconnecting linkage system, made operative by said aircraft r aching a preselected pressure altitude for opera said last-named means to maintain the absolute pressure within said compartment at a value above the ambient pressure at all practical flight altitudes within the operating limits of said control, said control means regulating the absolute pressure in such a manner as to change said pressure in inverse straight-line proportion to changes in the pressure altitude of said aircraft above the preselected pressure altitude; means for preselecting the pressure altitude at which said control means is made operative; and means for altering the proportion, said altering means being operative independently of said preselecting means whereby said proportion may be altered during flight of said aircraft without altering the preselected pressure altitude at which said regulating means is made operative.

3. An aircraft compartment pressure control comprising: means for delivering air to said compartment under a pressure greater than ambient flight pressure; means for discharging air from said compartment; means for varying the rate of air discharge from said compartment relative to the rate of air delivery to said compartmentwhereby the absolute pressure within said cabin may be varied; means, including a first capsule subject to cabin absolute pressure and a second capsule subject to cabin difierential pressure coacting through an interconnecting linkage system, for controlling said last-named means in such manner as to change the absolute pressure in said compartment in inverse straight-line proupon the attainment of a predetermined differential of cabin absolute pressure above extent absolute pressure fixed by the structure of the aircraft to override both of said controlling means and operate said varying means to maintain such diiferential as long as external absolute pressure is such as to tend to increase the differential.

4. An aircraft compartment pressure control comprising: means for delivering air to said compartment under a pressure greater than ambient flight pressure; means for discharging air from said compartment; means for varying the rate of airdischarge from said compartment relative to the rate of air delivery to said compartment whereby the absolute pressure within said cabin may be varied; means, including a first capsule subject to cabin absolute pressure and a second capsule subject to cabin differential pressure coacting through an interconnecting linkage system, for controlling said last-named means in such manner as to change the absolute pressure in said compartment in inverse straight-line [proportion to changes in flight pressure altitude; means for controlling said varying means for inhibiting the time rate of change of said cabin absolute pressure during changes of altitude of said aircraft which produce a rate of cabin pressure change in excess of said preselected rate of change; means for preselecting the rate of change of said cabin pressure at which said lastnamed controlling means is made operatve; means made operable by the attainment of a predetermined diilerential of cabin absolute pressure above external absolute (pressure for overriding both of said aforesaid controlling means and thereafter operating said varying means to maintain said predetermined diiferential substantially constant as long as external absolute pressure is below a predetermined minimum value; and means operable in accordance with a selected ratio between cabin absolute pressure and external absolute pressure, also operatively connected to said varying means, to control cabin absolute pressure so as to maintain said ratio substantially constant so that said air delivery means is operated within its capabilities throughout even the highest altitude range of the aircraft.

5. An aircraft compartment pressure control comprising: means for delivering air to said compartment under a pressure greater than ambient flight pressure; means for discharging air from said compartment; means for varying the rate of air discharge from said compartment relative to the rate of air delivery to said compartment whereby the absolute pressure within said compartment may be varied; control means, including a first capsule subject to cabin absolute pressure and a second capsule subject to cabin differential pressure coacting through an interconnecting linkage system, for operating said last-named means to maintain the absolute pressure within said compartment as a straight-line function of flight pressure altitude; a time-rate-of-pressurechange means operable at a preselected rate of change to preclude the normal action of said control means whenever the change in pressure altitude of the aircraft causes said control means to change cabin absolute pressure at a rate in excess of the preselected rate; means for preselecting said rate of change at which said means is operable; and means carried by said time-rateof-pressure-change means for rendering said control means inoperable and for rendering said time-rate-of-pressure-change means operable to supersede the action of said control means,

said compartment; means for varying the rate of air discharge from said compartment relative to the rate of air delivery to said compartment whereby the absolute pressure with said cabin may be varied; means, including a first capsule subject to cabin absolute pressure and a second capsule subject to cabin difierential pressure coacting through an interconnecting linkage system, for controlling said varying means in such a manner as to change the absolute pressure in said compartment in inverse straight-line proportion to changes in pressure altitude of the aircraft; normally inoperative rate of pressure change means, made operative by the aircraft ascending or descending at a rate such as to cause operation of said primary control which produces changes in the absolute pressure of the compartment in excess of preselected rates of change of cabin pressure, for controlling said varying means to maintain the rate of pressure change within the cabin to said preselected rate of change; and means for preselecting said rates of change of cabin pressure.

7. An aircraft compartment pressure control comprising: means for delivering air to said compartment under a pressure greater than ambient flight pressure; means for discharging air from said compartment; means for varying the rate of air discharge from said compartment relative to the rate of air delivery to said compartment whereby the absolute pressure within said cabin may be varied; means, including a first capsule subject to cabin absolute pressure and a second capsule subject to cabin differential pressure coacting through an interconnecting linkage system, for controlling said varying means in such a manner as to change the absolute pressure in said compartment in direct proportion to changes in pressure altitude of the aircraft; rate of pressure change means, made operative by the aircraft ascending or descending at a rate such as to cause operation of said controlling means which produces changes in the absolute pressure of the compartment in excess of preselected rates of change of cabin pressure, for controlling said varying means to maintain the rate of pressure change within the cabin to said preselected rate of pressure change; and means for manually adjusting said rate of pressure change means to cause said rate of pressure change means to control the rate of cabin pressure change independently of the rate of change of the pressure altitude of the aircraft.

8. An aircraft compartment pressure control comprising: airflow means for circulating the flow of air under pressure through the compartment and including pressurizing inlet means, outlet means, and valve means for controlling the discharge of air through said outlet means; means for regulating said valve means so as to control the absolute pressure within said compartment as a straight line function of pressure altitude of said aircraft; and anticipating control means sensitive to transient changes in diflerential between quantities of flow in said inlet and outlet means, respectively, adapted to produce in said regulating means a response to said transient 27 changes before any substantial change-in the absolute pressure of the cabin can occur as a result of said transient changes.

9. An aircraft compartment pressure control comprising: airflow means for circulating a flow of air under pressure through the compartment, said airflow means including inlet, outlet, and valve means for controlling said flow; means responsive to both cabin absolute pressure and flight pressure altitude so organized and arranged that the cabin absolute pressure is controlled to change at a predetermined ratio to the variation of pressure altitude of the aircraft from a preselected pressure altitude through regulation of said valve means, said valve controlling means including means defining a pressure chamber of restricted volume; and anticipating control means adapted to transmit to said chamber a pressure derived as a difierential result 01 the flows in said inlet and outlet means, respectively, whereby to normally maintain in said chamber a pressure equal to cabin pressure and varying in step with cabin pressure in response to gradual changes in said inlet and outlet flows but eflective in response to transient differences in said flows to change the pressure in said chamber in anticipation of corresponding changes in the pressure of the air within the compartment and to thereby actuate said valve controlling means to effect a corrective adjustment of said valve means so as to anticipate and prevent said corresponding changes in pressure in cabin air.

10. An aircraft compartment pressure control comprising: means for delivering air to said compartment under a pressure greater than ambient flight pressure; means for discharging air from said compartment; means for varying the rate of air discharge from said compartment relative to the rate of air delivery to said compartment whereby the absolute pressure within said cabin may be varied; means, including a first capsule subject to cabin absolute pressure and a second capsule subject to cabin diiferential pressure coacting through an interconnecting linkage system, made operative by said aircraft reaching a preselected pressure altitude for controlling said last-named means, thereby regulating the absolute pressure in said compartment to change said pressure in inverse straight-line proportion to changes in pressure altitude of said aircraft above the preselected pressure altitude; means for preselecting said pressure altitude; means including manually operable means for changing said proportion, said changing means being operable independently of said preselecting means whereby said proportion may be varied during flight of said aircraft without altering the said preselected pressure altitude at which said regulating means is made operative; and means controlled by operation of said manually operable means for indicating at any of the selected proportions the absolute pressure within said cabin as simulated cabin pressure altitude at any particular pressure altitude of said aircraft.

11. In an aircraft: a cabin adapted to be supercharged to maintain pressure in excess of that oi. the ambient atmosphere; means for supplying air to said cabin; means including a valve for discharging air from said cabin; means for controlling said valve to normally control cabin absolute pressure to change as a direct ratio of changes in the pressure altitude of the aircraft; time-rate-of-pressure-change means for supervising and vetoing control action instituted by the first said means; control means for overriding the action of both the aforementioned means and limiting the maximum cabin differential pressure to predetermined safe limits; means constituting an air flow conduit between said air supplying means and said air discharge means including a portion wherein pressure is higher than cabin pressure and another portion wherein pressure is lower than cabin pressure; means respectively flow-connecting a point in said conduit wherein the pressure is normally equal to cabin pressure to said ratio-to-flig'ht cabin pressure control means and to said overriding pressure control means; and means flow-connecting said timerate-of-pressure-change control means to said conduit adjacent said lower pressure point, whereby to make said pressure control means highly sensitive to air flow surges and maintain smooth automatic regulation of cabin pressure independent of transient changes in the air flow in said delivery and discharge means.

12. In an aircraft having a cabin adapted to be pressurized: a pressure discharge valve in said cabin; means for operating said valve; a cabin pressure regulating means for controlling said valve operating means; an electroresponsive means operable when energized to actuate said operating means to open said valve; a landing gear carried by said aircraft comprising two relatively movable components, one of said components adapted to engage the ground; circuit means connected to said electroresponsive means and including normally open switch means closed when said last named component is in engagement with the ground whereby said electroresponsive means is energized when the gear component is engaging the ground and the valve opened to obviate pressurization of the cabin due to inadvertent setting of said regulator below the pressure altitude of the landing field from causing cabin pressurization prior to flight and for equalizing cabin absolute pressure with ambient absolute pressure upon landing of the aircraft; and means including a time-rate-of-pressure-change regulating means for limiting the rate of change of cabin absolute pressure upon landing of the aircraft to prevent said pressure from decreasing in excess of some predetermined rate of change.

13. An aircraft compartment pressure control comprising: a pressure discharge valve including actuating means associated therewith; cabin pressure regulating means for operating said actuating means; an electro-responsive means operable when energized to control said actuating means and to render said regulating means inoperative; means responsive to the temperature within said compartment; switch means controlled by said temperature responsive means and connected to' energize said electro-responsive means upon the temperature of the cabin exceeding a predetermined temperature to render said regulating means inoperative and to operate said valve actuating means to open said valve; and

means for supervising and limiting the valve action to limit the rate of pressure decrease in the compartment.

14. An aircraft compartment pressure control comprising, means for delivering air to said compartment under a pressure greater than ambient flight pressure; means for discharging air from said compartment; means for varying the rate of air discharge from said compartment relative to the rate of air delivery to said compartment whereby the absolute pressure within said cabin may be varied; .means for controlling said last- 

